1. Field of the Invention
The present invention relates generally to the field of protective circuit arrangements for protecting circuit elements connected to a telephone line against the damaging effects of transient abnormal high voltage surges on the telephone line, and particularly, to an improved surge protection Data Access Arrangement (DM) for protecting against surges.
2. Background Art
The Underwriter Laboratory (UL) and the Federal Communications Commission (FCC) require devices that are connected to the public switched telephone network (PSTN), such as modems, to conform to specific rules. For example, UL standard UL 1459 and FCC part 68 set forth requirements for devices that interface to the PSTN.
The FCC requirements include tests where a voltage and current are applied to a device under test (also commonly known as a unit under test). The tests are designed to determine how well the tested device protects against the applied voltage and current (also referred to hereinafter as a surge).
In the FCC tests, an important protection criteria is commonly referred to as "metallic" or "differential mode" protection. Metallic protection is measured across the ring lead and tip lead of the telephone line.
Furthermore, the FCC tests can specify one of two compliance standards: an operational compliance standard (also known as a "non-destructive" compliance standard) and a non-operational compliance standard (also known as a "destructive" compliance standard). If the test specifies an operational compliance standard, the device under test must be operational after the test to pass the test. If the test specifies a non-operational compliance standard, the device under test can be non-operational after the test and still pass the test, as long as it meets certain impedance criteria.
Presently, the FCC requires all devices that are connected to the telephone network to pass a metallic voltage surge test. This metallic voltage surge test applies a 800V, 100A surge to the device under test to simulate a lightning strike. Since this is a metallic test, the surge is applied between the tip and ring (T&R) leads of a unit under test, such as a modem. The present metallic voltage surge test also specifies a non-operational compliance standard so that the unit under test passes the test even if the device is no longer operational after the test.
There are two conventional approaches to address the present metallic voltage surge test. The first approach employs a fuse or a fuse-like link, such as coils and resistors, to protect against the surge. The fuses or fuse-like links blow open whenever high currents flow through them. Once these fuses or fuse-like links are blown, the modem is inoperable. However, since the present metallic voltage surge test specifies a non-operational compliance standard, these devices would still pass the test.
The second approach employs an energy absorbing device, such as spark gaps, metal oxide variable resistors (MOVs), and SIDACTORS.TM. (manufactured by Teccor Electronics, Inc.), to absorb the surge.
The FCC is in the process of approving a new additional metallic voltage surge test that specifies an operational compliance standard. This new test applies a 1KV, 25A surge to the tip and ring leads of the device under test. In order to pass this test, the device must remain operational after the test.
Consequently, the prior art protection circuits that employ fuse-like links are inadequate to pass this new test because these circuits are not operational after the test. In addition, the prior art protection circuits that employ fuses or fuse-like links are not reliable since the fuses or fuse-like links may blow during normal operation when the unit is not undergoing a test surge. In other words, the fuses or fuse-like links may be sensitive to surges experienced during normal operation.
The prior art protection circuits that employ fuses or fuse-like links are also disadvantageous because of the 18-ohm resistors (connected in series with the tip and ring leads) that are typically provided with such circuits for surge protection. Under normal operation, the tip and ring resistors degrade modem signals by inducing a loss in energy. For example, to get a -10dBm output at tip and ring, the controller needs to transmit a -9dBm signal. This causes the controller to consume more energy than it really needs to in order to transmit a higher signal, and also causes the controller to be closer to saturation or distortion levels. These tip and ring resistors also render the overall circuit bulkier and more expensive.
In addition, the prior art protection circuits that employ fuses or fuse-like links compromise product reliability because voltage surges of less than, for example, 400 Volts but greater than, for example, about 150 Volts can cause the modem's tip and ring resistors to open up and disable the modem, just as if a large lightning strike had actually hit the wires leading to the modem. Therefore, since the energy absorbing device will protect the circuit against large voltage spikes only, the smaller spikes may still damage the circuit.
The prior art protection circuits that employ surge energy absorbing devices alone may not be sufficient to pass the new FCC test. Surge energy absorbing devices may fail the new FCC test because they are over-voltage devices, which means that they will not absorb the smaller voltage surges that may actually damage the circuit.
Accordingly, there remains a need for an improved surge protection circuit that passes the new FCC 1KV, 25A, non-destructive, surge test by ensuring that the unit under test is operational after the test, while providing an improved surge protection circuit that reduces the number of component parts, allows for higher integration, reduces the cost of the device, and maximizes the reliability of the device.